Problem: $\dfrac{9}{10} + \dfrac{13}{100} = {?}$
The fractions must have the same denominator before you can add them. Rewrite $\dfrac{9}{10}$ as $\dfrac{90}{100}$ $\dfrac{90}{100} + \dfrac{13}{100} = {?}$ $ = \dfrac{103}{100}$